Commonly assigned U.S. Pat. Nos. 6,349,116; 6,476,719; 6,434,194; 6,812,839; and 6,853,687, the disclosures which are hereby incorporated by reference in their entirety, disclose different types of compact transmitter units such as contained in a “tracking” tag (radio frequency (RE) tag transmitter), which is typically affixed to an object or asset to be tracked. These tags are operative with a digital detector/demodulator unit used for varying the “blinking” rate and other functions.
These patents describe a location system in which tag transmitters are configured to transmit or “blink” repeatedly for a short duration a wideband, spread spectrum pulse of RF energy that is encoded with the identification of an associated object and other information stored in a tag memory. Tag emission readers are geographically distributed within and/or around an asset management environment that contains the assets or objects, whose locations are monitored on a continuous basis and repeated to an asset management database, which can be accessible by a computer workstation or personal computer. The tag emission readers monitor the asset management environment for RE signals emitted from tags that are affixed to the objects to be tracked.
The blinks are bursts of RF energy emitted by the tags and monitored by the readers, which are installed at fixed and relatively unobtrusive locations within and/or around the perimeter of the environment being monitored, such as doorway jams, ceiling support structures and the like. A processor is connected to each tag emission reader and correlates the spread spectrum RF signals received from a tag with a set of spread spectrum reference signal patterns to determine which signals are first-to-arrive signals. A location processor is connected to tag emission readers and uses time-of-arrival differentiation of the detected first-to-arrive transmissions. The location processor uses a multi-lateration algorithm and locates the tagged object on a resolution as defined as about 10 feet.
It is possible to place short range, magnetic field proximity-based, tag-programming “pingers” at a respective location of the monitored environment, such as at a doorway through which a tag passes. This tag-programming “pinger” emits an AC magnetic field encoded with information, for example, programming information, data or “stimulus” that is coupled to any tag passing through that region. This information could be a reprogramming message this is operative to cause the tag to begin blinking at an increased rate for a brief period of time to alert the tracking system of the presence of the tag in that particular region. Alternatively, the tag transceiver could be modified to incorporate a magnetic field sensor that enables the tag to respond to the encoded AC magnetic field generated by the “pinger.” In one sspect, this “pinger” can generate a non-propagating AC magnetic field modulated with a frequency shift keyed (FSK) modulation signals representative of digital data to be transmitted to the tag.
As disclosed in some of the incorporated by reference patents, a receiver includes an LC tank detector circuit and magnetic field-sensing coil in parallel with a capacitor. The LC tank circuit resonates at a frequency between the two FSK frequencies used by the transmitter. The resonator detector is coupled to a sense amplifier that amplifies the voltage produced by the tank circuit for a desired receiver sensitivity. It buffers the detected voltage to a level used by a digital receiver-demodulator.
The digital receiver can be referenced to a clock frequency that corresponds to the difference between the two frequencies of the selected FSK modulation pair. In one aspect, the receiver contains two signal buffer paths, which operate on alternate sample periods one-half the period of the received data symbol rate, such that at least one of the two buffer paths will not be sampling data during transitions in the received FSK frequency. This demodulated data is buffered so that it may be clocked out to control circuitry in the tag. The tag can include an oscillator coupled to a variable pseudorandom (PN) pulse generator and other circuits as described in the above-identified and incorporated by reference patents.
The receivers in such tags and systems use frequency shift keying based circuitry, which often includes an FSK discriminator. If this type of receiver is used, however, receiver could fail to function in the presence of on-tone jammers.